Zeolitic materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion. It is often advantageous to dealuminate these materials in order to improve their process performance. Performance measures typically improved include product selectivity, product quality and catalyst stability.
Conventional techniques for zeolite dealumination include
hydrothermal treatment, mineral acid treatment with HCl, HNO.sub.3, and H.sub.2 SO.sub.4, and chemical treatment with SiCl.sub.4 or EDTA. The treatments are limited, in many cases, in the extent of dealumination by the onset of crystal degradation and loss of sorption capacity. U.S. Pat. No. 4,419,220 to LaPierre et al discloses that dealumination of zeolite Beta via treatment with HCl solutions is limited to SiO.sub.2 /Al.sub.2 O.sub.3 ratios of about 200 to 300 beyond which significant losses to zeolite crystallinity are observed.
U.S. Pat. No. 3,442,795 to Kerr et al. describes a process for preparing highly siliceous zeolite-type materials from crystalline aluminosilicates by means of a solvolysis, e.g. hydrolysis, followed by a chelation. In this process, the acid form of a zeolite is subjected to hydrolysis, to remove aluminum from the aluminosilicate. The aluminum can then be physically separated from the aluminosilicate by the use of complexing or chelating agents such as ethylenediaminetetraacetic acid or carboxylic acid, to form aluminum complexes that are readily removable from the aluminosilicate. The examples are directed to the use of EDTA to remove alumina.
EP 0 259 526 B1 discloses the use of dealumination in producing ECR-17. The preferred dealumination method involves a combination of steam treatment and acid leaching, or chemical treatments with silicon halides. The acid used is preferably a mineral acid, such as HCl, HNO.sub.3 or H.sub.2 SO.sub.4, but may also be weaker acids such as formic, acetic, citric, oxalic, tartaric acids and the like. Dealumination of zeolite Beta is not disclosed.
Therefore, it is an object of the present invention to provide a process for dealumination of a zeolite having the structure of zeolite Beta without significant loss of crystallinity. It is a further object of the present invention to increase the silica to alumina ratio in a zeolite having the structure of zeolite Beta. It is a further object of the present invention to improve process performance of a zeolite having the structure of zeolite Beta.